An analytical formulation of isotope fractionation due to self-shielding

摘要

Isotope fractionation due to photochemical self -shielding is believed to be responsible for the enrichment of inner solar system planetary materials in the rare isotopes of carbon, nitrogen, and oxygen relative to the Sun. Self -shielding may also contribute to sulfur isotope mass -independent fractionation in modern atmospheric sulfates, although its role in the early Earth atmosphere has not yet been convincingly established. Here, I present an analytical formulation of isotopic photodis- sociation rate coefficients that describe self -shielding isotope signatures for 3 and 4 -isotope systems broadly representative of O and S isotopes. The analytic equations are derived for idealized molecular spectra, making an analytic formulation tract- able. The idealized spectra characterize key features of actual isotopologue spectra, particularly for CO and SO 2 , but are applicable to many small molecules and their isotopologues. The analytic expressions are convenient for evaluating the mag- nitude of isotope effects without having to pursue involved numerical solutions. More importantly, the analytic expressions illustrate the origin of particular isotope signatures, such as the previously unexplained large mass -dependent fractionation associated with photodissociation of optically -thick SO 2 . The formulation presented here elucidates the origin of some of these important isotopic fractionation processes. (C) 2020 Elsevier Ltd. All rights reserved.